12V split charge system ideas

[normspanners]

Aux battery was charged with a split charge diode (70's technology )

the old split charge diodes dont charge properly either, because of the volt drop across the diode, the battery connected through the diode is always lagging by at least .8 volt ish so never achieves full charge.

 

[Robotstar5]

the old split charge diodes dont charge properly either, because of the volt drop across the diode, the battery connected through the diode is always lagging by at least .8 volt ish so never achieves full charge.

I know, but it were the best we afford in them days , and it's a 0.7 volt drop across the diode

 

[normspanners]

and it's a 0.7 volt drop across the diode

but you then have the associated wiring and terminals each causing volt drop, that is why I said it is at LEAST 0.8 volt, when actually measured, it usually suffers a LOT more on making the trip to the actually battery.

 

[Robotstar5]

but you then have the associated wiring and terminals each causing volt drop, that is why I said it is at LEAST 0.8 volt, when actually measured, it usually suffers a LOT more on making the trip to the actually battery.

If terminations are made correctly, volt drop will be minimal. My batteries were close together so didn't have any problems.

 

[gordon stephenson]

I ran a big Vauxhall estate that we used a lot of camping, Fitted 2 batteries and just hard wired both in parallel, it was like that for ten years and never had a problem

 

[normspanners]

Split Charge Diodes are dated and do not work properly.

The case against normal diode splitters

In simple terms diodes operate by only allowing current flow in one direction. A sort of one-way valve for electricity. Therefore 2 diodes could be connected from an alternator output terminal, one to each of 2 battery banks. The alternator can thus charge each battery bank but no current can flow in the opposite direction so loads connected to one battery cannot affect the other.

Seems like a simple solution?

If diodes really were a simple "one way valve for electricity" then, for many installations (not all), they would be an almost perfect solution to split charging multiple battery banks. However, real world diodes are not perfect "one way valves", they are anything but perfect .......

One of the side effects of diodes is that there is a voltage drop associated with them. This drop is not constant and varies from around 0.6 volts to 1.1 volts depending upon how much current is flowing through them. At very high charge currents the voltage drop can actually be as much as 1.6 volts. The higher the current, the higher the voltage drop. This means that the output voltage of your alternator at say 14 volts is not being presented to the batteries. The batteries will only be "seeing" between 12.9 and 13.4 volts depending upon the charge rate. Obviously this drastically reduces charging capability. The charge voltage on the batteries is simply too low.

Anyone who tells you that the voltage drop across a diode is fixed at 0.6 volts and does not change does not understand the subject and possibly read "beginners guide to electricity" and then stopped studying. Sorry, but that's the reality.

One solution is an external alternator regulator which senses the voltage at the battery instead of at the alternator (as per a normal alternator). This has the effect of compensating for the voltage drop across the diode by increasing the alternator output voltage.

Even this solution has it's problems. Firstly one has to decide which battery to sense. The domestic bank or the engine battery ? Let's put that to one side for the moment.....

Remember the voltage drop across the diode depends upon the current through it so the engine battery (which will usually be drawing much less current than the house bank) diode will have a lower voltage drop. Let's say it's 0.6 volts. The voltage drop across the diode to the domestic bank will be much higher (due to the higher charge current). Let's say it's 1.1 volt. We now have 0.5 volts difference between the battery terminal voltages.

Back to which bank to sense.... If the engine battery is sensed then we find that the domestic bank is being charged 0.5 volts too low. Obviously not what we are aiming for. If the Domestic bank is sensed we will find that the engine battery is being charged 0.5 volts too high. This of course will damage the battery.

Quoted from http://www.smartgauge.co.uk/diodes.html

 

[Mee]

Here you go...correction its called Durite.

Durite are fine but they are triggered by a specific voltage and they have a limited current capacity, at present the maximum standard Durite systems put out is around 20 amps maximum. Their main advantage is simplicity and if you can deal with a maximum of 20 amps to charge an auxiliary battery, they're fine.

Modern split charging systems have moved on and the programmable split charge systems are brilliant as you can set the split and you can set the charge rates between the batteries, most have some sort of warning system for low batteries and a secondary failsafe system for the engine battery which biases all he available charge to the main battery as the primary battery. Off road winching guys love them as they can run to an event and have a fully charged main battery and switch the bias to the maximum which is around 80-90% of the charging current to the auxiliary battery so it can charge at a much higher rate of up to 100 amps if it will accept it and they can winch for much longer without their batteries rapidly discharging.

Programmable charging systems have another little trick, Normspanners is correct in his claims about charging heavily discharged batteries and the way round this long trickle period before a battery will accept a decent amount of charge is to increase the voltage and reduce the current and modern programmable systems have the capability to raise the voltage way above the maximum 14.5 (approx.) voltage that a conventional alternator or battery charger will provide as many can raise the voltage to around 20 volts without damaging the battery. By increasing the voltage to its optimum (which varies) you can get the battery to accept a higher charging amperage much sooner, thus quicker charge times.

 

[rtbcomp]

Sorry but you are both wrong, a battery that is below 60 % charged will not be charged properly by the alternator,and if coupled together the higher charged battery will prevent the second battery receiving full charge because the higher voltage battery will limit the charge given by the alternator. thus not charging the lower condition battery
most batteries on vehicles are only 80% charged at best. because the vehicles are not run long enough in normal day to day running to charge them properly.

https://www.optimabatteries.com/en-...nators-are-not-designed-charge-dead-batteries

If a charged and discharged battery are connected together the voltage will be the same at each battery's terminals and at the alternator (sensing input, whether that's internal or external). The alternator cannot detect that it has a pair of batteries with different charge levels connected to it.

I'm happy to believe that an alternator will not charge a flat battery but that is not what we have here, we are discussing split charge systems with two batteries connected together.

"most batteries on vehicles are only 80% charged at best. because the vehicles are not run long enough in normal day to day running to charge them properly."

That's got nothing to do with an alternator's ability to charge a flat battery, that's because the alternator doesn't put enough ampere-hours back into the battery after starting the engine. Topping up with a battery charger will only have a short-term effect, the extra 20% will probably be used up at the next start and then you're back to the level the alternator keeps the battery charged to.

 

[rtbcomp]

One point not mentioned - fuses.

Connect a fuse at each end of the wires connected to the battery +ves, as close to the battery as possible.

 

[Exuptoy]

I think I mentioned fuses in post #2

 

[rtbcomp]

^^ Indeed, but not how many or where to connect them

 

[123hotchef]

Ok the battery ran flat because I was in the field with the horses with the 2 led light bars on, and as I have had a week off the truck has not had any decent runs to charge the battery because this is not usually a problem. After it happening once I dont want it to happen again, this time I was lucky I stole the electric fence energizer battery to jump the truck.

Ok so back onto the split charge system and wiring I have fitted a Aux fuse and relay board so running the lights ect from the 2nd battery is not a problem, I will take a feed off of the relay output to the Aux fuse board.

I am fully aware of fusing the cable to the 2nd battery which will be located in the rear or the truck bed. I made need 2 fuses or breakers one on each end.

Thanks for all the input so far people. It is good reading.

 

[Robotstar5]

...I am fully aware of fusing the cable to the 2nd battery which will be located in the rear or the truck bed. I made need 2 fuses or breakers one on each end.

You will only need protection (fuse/breaker) at the supply end.

 

[123hotchef]

You will only need protection (fuse/breaker) at the supply end.

yes but if i switch it over to start the truck the other end will become the supply end

 

[rtbcomp]

You will only need protection (fuse/breaker) at the supply end.

You've got 2 batteries, the cables need protecting from either or both batteries feeding a short to earth somewhere along the wire.

 

[123hotchef]

You've got 2 batteries, the cables need protecting from either or both batteries feeding a short to earth somewhere along the wire.

yep thats why both ends need a fuse as close to the batteries as possible

 

[normspanners]

I am fully aware of fusing the cable to the 2nd battery which will be located in the rear or the truck bed. I made need 2 fuses or breakers one on each end.

that will be a problem because of the length of cable causing volt drop I had exactly the same set up in my recovery transit professionally fitted and the rear battery for the halogen hazard light bar that could be running for hrs, ther battery didnt charge properly and didnt last because it used to sulphate,the batteries i used where truck sized, they only lasted a matter of months, in the end I fitted two alternators coz it was easy and cheap that cured the problem and the batteries lasted for years

If a charged and discharged battery are connected together the voltage will be the same at each battery's terminals and at the alternator (sensing input, whether that's internal or external). The alternator cannot detect that it has a pair of batteries with different charge levels connected to it.

that is the problem !! the low battery will not charge properly, because the higher voltage battery will control the charge !! that is why trucks have problems with the first battery that a radio is connected to, even in a 24v system because just a radio will drop the voltage enough to prevent the battery from charging correctly

 

[ajlelectronics]

that is the problem !! the low battery will not charge properly, because the higher voltage battery will control the charge !! that is why trucks have problems with the first battery that a radio is connected to, even in a 24v system because just a radio will drop the voltage enough to prevent the battery from charging correctly

That is purely because the resistance of the front to back cable comes into play. Two identical batteries connected in parallel, with equal length cables will behave as one large battery and charge to the same level.

 

[normspanners]

If you go onto boating forums charging extra batteries is a major problem and people spend thousands to get over those problems I have gone through this double battery scenario many times if you speak with specialist in the subject it is VERY complex.

 

[123hotchef]

I never had this problem in my cars years ago with batteries in the boot for big ICE installs